roms: rename x variable to it in for loop

there are two for loops that use x as a variable anme,
and an idiosyncrasy of certain sh implementations is
that these become global;

the result in this case was that when you finish building
every target in "./build roms", it would print "libgfxinit"
repeatedly, comma separated, instead of a comma-separated
list of the targets that were built.

work around it by renaming the variable in one of the loops.

Signed-off-by: Leah Rowe <leah@libreboot.org>